The invention relates to a tool making it possible to manufacture by moulding self-stiffened panels made from a composite material with a thermosetting matrix.
Composite material parts are generally produced by stretch forming several superimposed layers, each constituted by impregnated resin fibres. According to the envisaged application, the fibres can be of carbon, glass, Kevlar, etc. The resin constituting the composite material matrix is generally a thermosetting resin, which is thermally polymerized.
The invention is applicable to all industrial fields using composite material parts, i.e. particularly the aeronautical, space, car, maritime and railway industries.
In these different fields, the use of composite material parts makes it possible to make structures considerably lighter. This weight gain is due not only to the specific strength of these materials, which exceeds that of standard metal alloys, but also the possibility offered by these materials with regards to obtaining complex shapes by moulding. Therefore it is possible to assemble as a single composite material part a mechanical subassembly which, conventionally, would be constituted by several basic metal parts interconnected e.g. by welding or mechanical fixtures (rivets, screws, etc.).
In the particular case of self-stiffened panels formed from a base plate provided with stiffeners, it is possible to produce these panels as a single composite material part, whereas according to the prior art the panels are made from a base plate to which are connected, e.g. by riveting, angle iron-shaped stiffeners.
The manufacture of such self-stiffened panels made from composite material with a thermosetting matrix takes place by moulding using an appropriate tool, which is placed in an oven or an autoclave, whereof the temperature rise ensures the polymerization of the resin.
In practice, this moulding operation causes production problems linked on the one hand with the functions to be satisfied during the moulding of a thermosetting composite material and on the other with the special structure of self-stiffened panels.
The functions to be satisfied during the moulding of a thermosetting composite material part are:
the temperature rise necessary for activating the polymerization reaction; PA1 the application of a pressure perpendicular to the surface of the layers throughout the duration of the baking cycle of the part necessary for the good compacting of the layers and the final quality of the part; PA1 the application of a uniform pressure to the entire surface of the part, so as to avoid the local expulsion of the resin by a differential pressure effect between two zones, resulting from the low viscosity of said resin at the start of the polymerization cycle; and PA1 the use of a moulding tool, whose pressure application system is able to follow the thickness decreae of the parts in all directions occurring during polymerization, in order that said system can fulfil its function throughout the polymerization cycle.
Moreover, the manufacture of a self-stiffened panel requires the use of a moulding tool which is also able to fulfil the geometrical function of maintaining the spacing between the stiffeners and maintaining the planeity of said stiffeners, so as to prevent any undulation thereof, which might lead to a local buckling of the compressively stressed panels.
The presently used moulding tools for moulding such structures suffer from major disadvantages. A first known tool is constituted by a closed or sealed mould, in which are fixed inflatable bags positioned between the stiffeners of the panel to be polymerized. In this case, the mould containing the part and the bags must be solid, so as to take up the internal compressive stresses applied thereto. It is therefore expensive and difficult to heat.
Moreover, as inflatable bags are by their very nature flexible, floating elements, the maintaining of the spacing between the stiffeners and the maintenance of the planeity thereof are not ensured in a satisfactory manner. In other words, the parts obtained suffer from serious geometrical defects.
Finally, such a tool becomes very voluminous when used for moulding parts several meters long.
Another known tool for moulding self-stiffened panels has, like the previous one, a solid sealed mould. However, the inflatable bags are replaced by elastomer shims. This tool has the same disadvantages as the previous tool with regards to the dimensioning of the mould.
In addition, the pressure applied to the part results from the thermal expansion of elastomer cores, so that the pressure and temperature cannot be separately controlled. This is prejudicial when using certain resins for forming the composite material matrix.
Moreover, although the geometrical maintenance of the stiffeners is better than when using inflatable bags, the creation of local overstresses can lead to flow or creep of the material constituting the expandable cores, so that the latter then move with them the adjacent stiffeners. The maintenance of the spacing between the stiffeners, as well as their planeity are consequently not totally assured.
In addition, the solid elastomer cores do not make it possible to uniformly redistribute the stresses in all directions, so that the pressure applied to the part being moulded is not uniform. Finally, the thickness reduction of the part during polymerization is accompanied by a drop in the pressure applied by the elastomer cores, which has an effect on the final quality of the part.
In another known tool, elastomer shims are also placed between the stiffeners of the panel to be polymerized, but the thus formed assembly is placed between a block on which the base plate of the panel rests and a bag, whose peripheral edge is connected in sealed manner to the block, said bag covering the elastomer shims. The pressure is applied to the part by forming a vacuum in the space defined between the block and the bag and containing both the part and the shims. In this case, the elastomer shims are used as geometrical shapers and redistribute on the panel the internal pressure of the autoclave, which is applied to the sealing bag.
In this procedure, pressurizing is separate from the temperature and the same flexibility for the control of the baking cycles is obtained as when using inflatable bags. Furthermore, the tool does not have to take up high compressive stresses, because on both faces it is exposed to the pressure prevailing in the autoclave. Therefore the block can be formed in a very light wall.
However, as in the preceding arrangement, the pressure applied to the part is not perfectly uniform and the maintaining of the spacing between the stiffeners and the planeity of the latter are dependent on possible creep of the material constituting the shims.
In a fourth known method for producing self-stiffened composite material panels, use is made of a tool comparable to that of the third method described hereinbefore, i.e. a block associated with a sealing bag, whilst replacing the elastomer material shims by solid, non-deformable shims. The bag then applies the pressure prevailing in the autoclave, through the non-deformable shims, to the base plate parts located between the stiffeners. Moreover, the differential expansion between the shims and the block applies a pressure to the faces of the stiffeners. Thus, this method makes it possible to guarantee good positioning and planeity characteristics of the stiffeners.
However, the pressure applied to the stiffeners is dependent on the temperature and is consequently not identical to that applied to the base plate. Consequently there is a differential pressure between the base plate and the stiffeners, which tends to expel the resin towards the base plate at the end of the polymerization cycle.
Moreover, this solution is disadvantageous when the base plate has local overthicknesses. Thus, thickness variations of the plate during the polymerization cycle differ as a function of whether the base plate zones are thicker or thinner, so that the non-deformable shims would only bear at the end of the cycle on the thinner zones of the base plate. Consequently no pressure would then be applied to the thicker zones of the latter.
Thus, at present, there is no moulding method which is completely satisfactory for making it possible to produce self-stiffened panels made from a composite material with a thermosetting matrix.